If you rebuild your engine, you will know what you have in it. If you buy a good replacement engine, well, it could hand grenade the 2nd time you ride it. It will be used and you have no idea how the last owner or owners treated it. For a 110, you can purchase a 125 piston which is actually the last overbore on the 110 cylinder, so that's no biggie. Also you can purchase or make yourself a slightly custom exhaust and get yourself a really nice airfilter and install an aftermarket cam which will help pep up your performance a little which should help you keep up with your buddys 125. Just remember when freshening up your trike, do it right. Don't skimp on fixing the rear brake or your wheel bearings or rear chain as all those will affect how well your trike runs.
Nicholson 500x (Project)
Nicholson 185s
Nicholson 110 (That takes an atc70 tank)
Ascott 500 head/350x (Project)
Homebuilt Racer Chromoly Suspension 110 (Ragin Runt)
PK Racing Suspension 70
A+ Inc Suspension 70
AWS Aluminum 90 frame
Hi Performance ATC Suspension frame rd350
Hi Performance ATC Suspension frame 90